SAN FRANCISCO  The wind farm at Altamont Pass produces clean electricity for tens of thousands of homes. The windmills also kill a large number of migratory birds passing through the region east of San Francisco.

Two environmental groups are going to court with the ultimate aim of shutting down the farm for three months during the migration season to help protect red-tail hawks, golden eagles, and other raptors killed by the mills.

Friday, the nonprofit group Californians for Renewable Energy filed a suit in Alameda County Superior Court demanding the county conduct an environmental impact report on the windmills. CARE president Michael Boyd said Saturday the Golden Gate Audubon Society was filing a sister suit Monday.

The nearly 6,000 windmills on the rolling hills of Altamont Pass, 60 miles east of San Francisco, make up one of the largest wind farms in the country.

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The turbines generate enough electricity to power more than 120,000 homes. And more birds are killed in the region than at any other wind farm in the country, according to the California Energy Commission.

Earlier this year, the Alameda County Board of Supervisors and the companies that operate the turbines agreed to shut down 50 percent of the turbines during prime migration season, set to begin Tuesday, and to shut 100 mills permanently as part of a plan to protect the birds.

But Boyd said the plan did not go far enough and did not have merit because it was drawn up without conducting an environmental impact reporting required by the California Environmental Quality Act.

Under the act, when a suit is filed, all parties involved must sit down for a mandatory settlement conference. At that time, Boyd said, CARE will ask for all turbines to be shut down for three months during migration, and for 200 mills to be removed.

"The Alameda County Board of Supervisors believes they are exempt from CEQA because these turbines were built more than 20 years ago before they knew about the birds," he said.

A majority of the turbines are owned and operated by Florida Power & Light, among other companies. Neither Alameda County Board of Supervisors nor representatives from the companies involved could be reached for comment.